In a continuing study of factors that regulate the operation of the lower esophageal sphincter, studies will be carried out in esophageal smooth muscle of opossums and cats. Isolated strips of muscle, 1 cm x 1 mm x 1 mm will be studied by electromicroscopy to seek distinguishing features of nerve and muscle morphology that will correlate with established physiological differences between muscle of the sphincter and that of the esophageal body. Similar morphologic correlations with the function of this muscle will be sought by a special method to examine the gross morphology of the intramural plexuses. The hypothesis will be examined that the sphincter muscle differs from the esophageal body muscle in respect to the source of calcium responsible for the tonic contraction of the sphincter and for the place of calmodulin in the calcium activation process for sphincteric tone.